NoClassDefFoundError happens when using "function"s in drl

package org.drools.example.api.defaultkiesession.Hal1

import org.drools.example.api.defaultkiesession.Message

function boolean alwaysTrue() { 
	return true;
}

rule "rule 1" when
    m : Message( )
then
    retract(m);
end

rule "rule 2" when
    Message( alwaysTrue(), text == "Hello, HAL. Do you read me, HAL?" )
then
    insert( new Message("HAL", "Dave. I read you." ) );
end

package org.drools.example.api.defaultkiesession;

import org.kie.api.KieServices;
import org.kie.api.runtime.KieContainer;
import org.kie.api.runtime.KieSession;

public class DefaultKieSessionExample
{
    static int counter = 0;
    public void failCase1()
    {
        KieServices ks = KieServices.Factory.get();
        KieContainer kContainer = ks.getKieClasspathContainer();

        KieSession kSession = kContainer.newKieSession();
        for( counter = 0; counter < 10000; ++counter) {
            kSession.insert(new Message("Dave", "Hello, HAL. Do you read me, HAL?"));
            kSession.fireAllRules();
            if (kSession.getFactCount() != 0) {
                System.err.println("error");
            }
        }
    }

    public static void main(String[] args)
    {
        try {
            DefaultKieSessionExample defaultKieSessionExample = new DefaultKieSessionExample();
            defaultKieSessionExample.failCase1();
        } catch (NoClassDefFoundError error) {
            System.err.println("Failed after "+counter+" testcases");
            error.printStackTrace();
        }
    }

}

java.lang.NoClassDefFoundError: org/drools/example/api/defaultkiesession/Hal1/AlwaysTrue
	at ConditionEvaluator19fe4e382c304060b0046f5cdc6a59fa.evaluate(Unknown Source)
	at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:217)
	at org.drools.core.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:174)
	at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:134)
	at org.drools.core.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:502)
	at org.drools.core.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:387)
	at org.drools.core.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:288)
	at org.drools.core.reteoo.EntryPointNode.assertObject(EntryPointNode.java:260)
	at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:360)
	at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:279)
	at org.drools.core.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:1148)
	at org.drools.core.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:1092)
	at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:308)
	at org.drools.example.api.defaultkiesession.DefaultKieSessionExample.failCase1(DefaultKieSessionExample.java:17)
	at org.drools.example.api.defaultkiesession.DefaultKieSessionExample.main(DefaultKieSessionExample.java:29)
Caused by: java.lang.ClassNotFoundException: org.drools.example.api.defaultkiesession.Hal1.AlwaysTrue
	at org.drools.core.common.ProjectClassLoader.tryDefineType(ProjectClassLoader.java:123)
	at org.drools.core.common.ProjectClassLoader.loadType(ProjectClassLoader.java:114)
	at org.drools.core.common.ProjectClassLoader.loadClass(ProjectClassLoader.java:84)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	... 15 more